In the optical memory field, those used with optical disk systems, for example, separate lasers with different wavelengths are used for writing, reading, and tracking of the read/write laser spots on the disk and inside the track. The major advantages of such a system are good performance and versatility; while the major disadvantages are complicated optics, critical alignment, and they are bulky and expensive. One prior art type of laser recording apparatus utilizing a servo control to control the angular position of the beam splitter to obtain a maximum readout signal from the beam splitter is disclosed in U. S. Pat. No. 3,474,457, entitled "Laser Recording Apparatus", which issued on Oct. 21, 1969. FIG. 1 of the patent discloses that a portion of the incident laser beam is reflected back from the information recording medium and a readout beam splitter is used to focus the beam onto a photodetector. The output of the photodetector produced by the reflected portion of the laser beam from the beam splitter provides an input to a servo controlled mechanism and operates to control the angular position of the beam splitter with respect to the laser beam to obtain the maximum readout signal from the beam splitter.
According to the present invention, a single semi-conductor laser is utilized in an optical system to record and read back from a recording media such as a spinning disk. By means of a retroreflecting light and optical system, the same optical path can be used not only for recording, but for read back of the recorded data on the recording media. A polarizing beam splitter is utilized to cause the light beam to pass through an optical system whereby both focus error and tracking error as well as the data signal can be generated. By dithering the light beam PG,3 across the recorded track in conjunction with the data signal, a tracking error signal is generated to cause the dithering motion of the applied galvanometer to maintain its center tracking of the recorded information. Similarly, by use of a separate beam splitter, a focus error signal can be detected by means of error signals from complementary detectors to cause a focusing lens to maintain at audio frequencies an accurate focus of the applied light beam on the recording media.